Currently, research and development efforts are making advances in technology for achieving high density optical recording that is capable of storing large quantities of information per unit of surface area. In products currently utilizing optical disk technology, a laser light is irradiated through a lens onto a disk, and the data recorded on the disk is then reproduced and/or recorded. Up until now, in order to record data at high densities, technology has been developed for reducing the size of the focused laser beam spot. The spot size was proportional to λ/NA, where the wavelength of the light source is λ and NA is the numerical aperture of the objective lens. In other words, much progress was made in storing larger quantities of information on one disk by enlarging the numerical aperture of the objective lens and shortening the light source wavelength. Here, the combination of the light wavelength, the NA of the objective lens, and the capacity for writing data stored on a medium that is 12 centimeters in diameter is 780 nm, 0.5, 650 MB, respectively, for a CD; and it is 650 nm, 0.6, 4.7 GB, respectively, for a DVD. Disclosed technology utilizing a blue laser source allows two combinations comprised of 405 nm, 0.85, 25 GB; and 405 nm, 0.65, 20 GB. This recording capacity is sufficient for recording approximately two hours of high definition television image data.
However, this recording capacity is insufficient for applications in security systems and business systems, such as broadcast stations. Such applications typically require a recording capacity, for example, of 100 GB or more per disk. Preferably, it should be possible to record as great a quantity of information as possible on one disk in view of the existence of locations which store large quantities of data on storage mediums, such as for image data requiring a long term storage ranging from a few dozen to a hundred years. The capacity required for such tasks ranges from several hundred GB to 1 terabyte (TB) or more.
However, increasing the disk storage capacity by utilizing the above-described methods is practically impossible. First of all, development of a semiconductor laser for a short wavelength light source is extremely difficult. Secondly, even if such a semiconductor laser were developed, the light source is ultraviolet light, which is absorbed by the disk substrate and protective film, and so obtaining a satisfactory write/read quality is predicted to be impossible. Research into enlarging the NA of the objective lens is progressing, and technology relating thereto was reported, for example, in the Japanese Journal of Applied Physics Vol. 42, pp. 1101-1104 (2003) involving the setting of the NA to 1.8. However since the light used for a write/read operation in this system is not the normal propagating light, but is a light within the lens referred to as near-field light, the lens must be brought extremely close to the disk surface, and that distance must be maintained between the lens and disk, so that a mechanism to move the lens over the disk is necessary. This type of system closely resembles the hard disk used in magnetic recording, and disk removability, which is one advantage of optical disks, is difficult in this system.
To resolve these difficulties, super-resolution technology was proposed for effectively improving the light resolution by installing some type of mechanism on the disk. Super-resolution technology utilizing a phase-change recording film as the medium was reported, for example, in the Japanese Journal of Applied Physics Vol. 32 pp. 5210-5213. Phase-change film is usually utilized as a recording film for rewritable disks, such as CD-RW, DVD-RAM, DVD±RW and Blu-ray discs, etc. However, this recording material is not used as a recording film, but rather, it is used as a layer to effectively boost the optical resolution, in the same manner as the readout layer on a magneto-optical (MO) disk. This type of layer is called a super-resolution layer. In this method, a phase-change recording film is formed by sputtering onto a read-only (ROM) disk, to melt a portion of the phase-change recording film during read-out. If the melted portion has a sufficiently higher disk reflectivity, then the signal obtained from the melted portion will be predominant among the readout signals. In other words, the melted portion of the phase-change film essentially becomes the readout light spot. The surface area of the melted portion is smaller than the light spot, so that the readout light spot size is reduced to improve the optical resolution.
In the method disclosed in patent document 1, a phase-change film is utilized as the super-resolution layer, and the film thickness of that phase-change film is modulated according to the recording pattern to establish a thin and a thick phase change film. In the disk fabrication for this method, first of all, a mask containing a recording mark pattern is formed by optical lithography. The film thickness of the phase-change film is then modulated, for example, by patterning by reactive ion etching by way of the mask during or after sputtering via the mask to form the phase-change film. By regulating the readout light power during readout of a disk fabricated in this way, only the thin section of the phase-change film melts without the thick section melting. In this way, a super-resolution effect can be obtained in the same way as the above-described method that uses a phase-change film as a super-resolution film.
Another type of super-resolution technology that is disclosed in patent document 2 attempts to improve the recording characteristics by recording signals on concave capsules. In this patent document 2, an array of concave pits of equal length are fabricated on the substrate, and a phase-change film, a protective film and a reflective film are formed on that substrate, and the crystalline-amorphous shape of the phase-change recording film of the pit is controlled by the same method as used in normal phase-change recording. These concave pits constitute the recording units. On an ordinary rewriteable phase-change medium, film thickness irregularities occur when rewriting data many times in the same location on the disk, and these irregularities cause the write/read characteristics to deteriorate. One reason for this deterioration is that the melting of the recording film, due to the scanning of the light spot during data rewriting, induces a temperature gradient in the melted section that causes a material flow in the recording film. In the method disclosed in patent document 2, however, the location melted on the recording film is within the concave pit, so that material flow of the recording film is inhibited and a larger number of data overwrites is possible. Besides super-resolution technology, multi-layer optical disk technology has also been disclosed for achieving optical disks with a large information capacity. In this technology, one optical disk contains multiple recording surfaces, and the recorded data is separately recorded and/or reproduced on these recording surfaces. This multi-layer optical disk technology, for example, has been reported in the Japanese Journal of Applied Physics Vol. 38 pp. 1679-1686. To record and reproduce using this method, first of all a light is irradiated from the substrate side of the disk in the same manner as with a conventional optical disk, and a light spot is focused on the recording surface for recording and reproducing. The quantity of reflected light is detected during readout. In other words, the light must transmit through the layer on the nearest side during recording/reproducing on the innermost layer, as seen from the substrate side. The structure of the conventional optical disk is designed to take reflectivity into account. However, in a multilayer optical disk, the transmittance must be maintained at a certain level or higher in all except the innermost layer. In other words, the structure of the multilayer optical disk must meet demands for both transmittance and reflectivity in each layer. Moreover, each layer must be separated by at least a certain distance from other layers to prevent effects from marks recorded on other layers during readout of a layer. This method is reported in the Proceedings of the SPIE, Vol. 5069, pp. 90-97, disclosing an example attaining a four-layer write-once optical disc (CD-R/DVD-R). The transmittance of each layer from the substrate side, as provided in that report, was, respectively, 81.6%, 74.4%, 63.3% and 0%. The amount of reflectivity obtained during readout from a disk drive was five percent or more for all layers.
[Patent document 1] JP-A No. 244870/1995
[Patent document 2] JP-A No. 282674/1993
[Non-patent document 1] Japanese Journal of Applied Physics Vol. 42 pp. 1101-1104 (2003)
[Non-patent document 2] Japanese Journal of Applied Physics Vol. 32 pp. 5210-5213
[Non-patent document 3] Japanese Journal of Applied 15 Physics Vol. 38 pp. 1679-1686
[Non-patent document 4] Proceedings of SPIE Vol. 5069, pp. 90-97
The above-described methods achieve a super-resolution effect mainly by using heat to form a region using the varied optical properties in the light spot. These methods effectively improve the optical resolution. These methods are also capable of obtaining a mark readout signal at a size where normal readout methods cannot obtain a sufficient signal. Moreover, these methods boost the recording capacity of the optical disk, or in other words, are capable of recording the data at a higher density.
However, the heat distribution within the light spot contains jitter. The reason for this jitter is the presence of crystalline particles caused by the fact that at least a number of films forming the disk are poly-crystal and/or that thermal diffusion uniformity is lost due to defects in the film. Due to these factors, jitter is present in the shape and size of the effective readout light spot formed by the super-resolution effect. This jitter is the cause of readout errors and noise that are not present when using normal readout methods. This type of noise is referred to here as super-resolution noise.
Except for the organic film, all of the super-resolution film is poly-crystal. The particle sizes of these films differ according to their materials, but they are generally several tens of nm or more. The molecular size of an organic film is also determined in those units and is approximately 10 nm. In other words, the effective spot size in super-resolution contains a jitter of several dozens of nm.
Spot size jitter in conventional disk drives occurs due to errors in the auto-focusing servo, etc. In conventional disk drives, the spot size jitter due to auto-focusing errors is set within approximately five percent. Therefore, in super-resolution, a five percent jitter is also allowed in the effective spot size. Moreover, assuming a heat jitter of about 20 nm due, for example, to crystalline particles, the required effective spot size is 400 nm or more. The Blu-ray disc currently available in product form possesses a light source wavelength of 405 nm, and the NA for the objective lens is 0.85, so that the λ/NA is approximately 480 nm. The Blu-ray disc recording capacity is about 23.3 gigabytes, so that the recording capacity achieved by super-resolution is (480/400)2 times that amount, approximately 33.5 gigabytes. This figure is the limit that the recording density can be improved in a medium by super-resolution technology. Therefore, achieving a several hundred GB to 1 TB recording capacity, as was described in the “Background of the Invention”, is impossible with conventional techniques.
There is an upper limit on the number of layers of recording surfaces in multi-layered optical disks, because the design must take the transmittance and the reflectivity of each layer into account, as described in the “Background of the Invention”. The reason for this is that, when one considers those layers that are mostly on the substrate side, for example, a high transmittance must be maintained because of the many layers. However, when the transmittance is set high, the reflectivity becomes low, and there is a decrease in the signal amplitude for that layer. Conversely, when the reflectivity is set high in order to improve the signal level for that layer, little light reaches the inner layer, so that there is a decrease in the signal amplitude of that inner layer.
In the method described in patent document 1, the film thickness of the phase-change film is modulated, however the recording film is continuous, so that there is a large possibility that a portion of the space section will melt during the melting of the mark section due to heat flowing to the space section. The melted surface area of this space section depends on the recording film particle size, defects within the recording film, and jitter in the transition section changing with the film thickness, causing the same potential problems described above that limit the super-solution effect.
In the method of patent document 1, the film thickness of the phase-change film is formed by sputtering using a mask or is formed by RIE (reactive ion etching) using a mask. In these methods, such as sputtering or RIE that utilize plasma, the plasma leaks from the rear side of the mask, so that the film thickness transition section becomes smoother. The width of this transition section depends on the process conditions, but is generally about 100 nm. The transition section is in front of and behind the mark, so using the method of patent document 1 is impossible when the mark length is 200 nm or less. In other words, it is difficult to utilize this method to attain a recording capacity equivalent to or greater than the capacity of the Blu-ray disc.
The method of patent document 2 can improve the overwrite cycle (the number of times rewrite can be performed) by suppressing the material flow during recording. However, when using this method during readout, the recording density is determined by the light spot size, as determined by the light source wavelength, and the NA of the objective lens. The method of patent document 2, therefore, renders no effect in improving the recording density or the recording capacity.